Hi, I want to build subwoofer amp using TDA7294 bridge application. My probem is that in datasheet is +-25V for 8 ohm load but is it voltage of transformer or voltage after rectification? Because now i dont know if i need to buy 18-0-18 trafo or 25-0-25 trafo. Thanks for help and sorry for my english.
Vs is DC value. You will be dissipation limited, so it's probably better to use a lower voltage anyway.
Meaning, in case it wasn´t clear, you will need a 18+18VAC transformer.
That said, I find the "150W into 8 ohm" rating optimistic, maybe achievable in their Lab with special supplies but not in a real world home build.
To begin with , unregulated raw supplies (transformer>bridge>capacitors, period) drop under load, so a +/-25V one will give you some 22.5+22.5V under load, and we are talking about a properly specified one.
Each chipamp output will get within 2V of that, so you end up with some 20.5V peak.
It being a bridge: 40.1V peak, so 28.35V RMS so 100W RMS
A far cry from "150W" although of course it´s a very usable power out.
That said, I find the "150W into 8 ohm" rating optimistic, maybe achievable in their Lab with special supplies but not in a real world home build.
To begin with , unregulated raw supplies (transformer>bridge>capacitors, period) drop under load, so a +/-25V one will give you some 22.5+22.5V under load, and we are talking about a properly specified one.
Each chipamp output will get within 2V of that, so you end up with some 20.5V peak.
It being a bridge: 40.1V peak, so 28.35V RMS so 100W RMS
A far cry from "150W" although of course it´s a very usable power out.
That´s a good transformer for a 8Ohm single chip amplifier but really "on the edge" for a bridge application.
In a bridge application each amp effectively sees a 4Ohm load so I wouldn´t go higher than 2x18VAC.
I used a 2x22VAC transformer for a TDA7293-amp and accidentally (sine wave) tested with a 4Ohm load.
The chip went into protection real quick and is practically not easy to cool at those voltages.
As Sangram says these chips are mostly "dissipation-limited".
Problem with TDA729x is also the relatively high thermal resistance from chip to case (~1.5KW).
Even with an ideal heatsink temperature rise can be plenty.
(quote from datasheet "the thermal resistance of the heatsink must be 0.038
°C/W" !!!)
Bridge application and single-chip for 4Ohms speakers I´d use 2x18VAC.
Single-chip for 8Ohms speakers 2x24VAC.
So 2x18VAC transformer would be usable with all applications and typical speaker impedances.
Don´t skimp on the heatsink. Even more important with these chips is spreading the heat quickly so a heatsink with a thick base or use thick aluminium plate between chip and heatsink.